Although the present invention is generally applicable to probing any electronic device, the present invention is particularly suited for probing an integrated circuit to test the circuit. As is known, integrated circuits are typically manufactured as a plurality of dice on a semiconductor wafer. FIG. 1 illustrates a typical test system 100 for testing such a semiconductor wafer 124. The exemplary test system shown in FIG. 1, includes a tester 102, a test head 118, a probe card 106, and a prober 120.
A semiconductor wafer 124 is placed on a chuck (also commonly referred to as a stage) 114, which typically is capable of movement in the “x,” “y,” and “z” directions. The chuck 114 may also be capable of being rotated and tilted and may be further capable of other motions as well. Once the semiconductor wafer 124 is placed on the chuck 114, the chuck is typically moved in the “x,” “y” directions so that terminals on the dice (not shown) of the wafer 124 align with probes 108 on the probe card 106. The chuck 114 then typically moves the wafer 124 upward in the “z” direction, bringing the terminals into contact with the probes 108. One or more cameras 122 may aid in aligning the terminals and the probes and determining contact between the probes 108 and the terminals.
Once the terminals of the dice (not shown) are in contact with the probes 108, a tester 102, which may be a computer, generates test data. The test data is communicated through one or more communication links 104 to a test head 118. The test data is communicated from the test head 118 through interconnections 116 (e.g., pogo pins) to the probe card 106 and finally to the terminals of the dice (not shown) through probes 108. Response data generated by the dice are communicated in reverse direction from the probes 108, through the probe card 106, through interconnections 116, through the probe head 118, through a communication link 104, to the tester 102.
FIGS. 2A–2C illustrate movement of the wafer 124 into contact with the probe card 106. As mentioned above and shown in FIG. 2A, terminals 204 of one or more dice 202a of wafer 124 are aligned with probes 108 of the probe card 106. The chuck 114 them moves the wafer upward such that the terminals 204 of the die 202a contact probes 108, as shown in FIG. 2B. As shown in FIG. 2C, the chuck 114 typically moves the wafer 124 beyond first contact with the terminals 204. (Movement beyond first contact is often referred to as “over travel.”) This typically compresses the probes 108. The resulting spring force exerted by the probes 108 against the terminals 204 helps to create a reasonably low resistance electrical connection between the probes and the terminals. In addition, the probes 108 often wipe across the surface of the terminals 204 as the probes are being compressed. The wiping action tends to cause the tips of the probes 108 to break through any oxide or other build up on the terminals 204, again helping to create a reasonably low resistance electrical connection between the probes and the terminals.
FIG. 3A illustrates over travel and the resulting compression and wipe of an exemplary probe 108 of probe card 106. While at a position representing first contact between the probe and the wafer, the probe, wafer, and terminal are labeled 108a, 124a, and 204a, respectively, and shown in dashed lines in FIG. 3A. While at a position representing over travel by a distance 304, the probe, wafer, and terminal are labeled 108b, 124b, and 204b, respectively, and shown in solid lines in FIG. 3A. The wipe of the tip of the probe 108 across the terminals 204 is labeled 302 in FIG. 3A.
As might be expected, compression of the probe 108 due to over travel induces forces and stresses in the probe. The types and locations of the forces and stresses depend on, among other things, the shape and design of the probe. A few exemplary forces and stresses induced in the probe 108 illustrated in FIG. 3A are shown in FIG. 3B. A principal stress is produced by a torque or rotational force 306b about the point at which the probe 108 is secured to the probe card 106. Of course many other forces and stresses may be induced in the probe. The spring force of the probe 108 may also exert a force 310b against the terminal (not shown in FIG. 3B) of the wafer.
Such forces and stresses may break, damage, or reduce the useful life of a probe 108. Also, the force 310b exerted by the probe 108 against the terminal may damage the terminal and/or the wafer. A wafer comprising material with a low “k” dielectric may be particularly susceptible to such damage. Generally speaking, the greater the friction between a probe 108 and a terminal 204, the greater such forces and stresses are likely to be. Indeed, it is possible for frictional forces to prematurely stop the wiping of the probe 108 tip across the terminal 204. This may happen, for example, if the probe 108 tip digs too deeply into the terminal 204 or if the probe tip gets caught in an irregularity on the surface of the terminal. If the probe 108 tip stops its wiping motion prematurely, the forces and stresses that build up on the probe may become particularly large (and therefore particularly likely to cause damage to the probe, terminal, and/or wafer). Although a probe 108 may dig into any type of terminal 204, a probe 108 is particularly susceptible to digging into a terminal made of a soft material (e.g., solder ball terminals) or a terminal with a rough surface (e.g., copper terminals).
FIG. 3C illustrates in dashed lines the position of a probe 108a while at initial contact with the terminal (not shown in FIG. 3C). (This is similar to the position labeled 108a in FIG. 3A.) Also, in dashed lines in FIG. 3C is the position 108b of the probe after over travel, which is similar to position 108b shown in FIG. 3C. In FIG. 3C, however, a position and compression of the probe where the tip dug into the terminal and stopped part way through its wiping motion is also illustrated and labeled 108c (and in solid lines). As can be seen, the probe is more deformed in position 108c than in position 108b (its position had it not dug into the terminal). Consequently, the forces and stresses 306c, 310c such as those shown in FIG. 3B, may be significantly greater in position 108c than in position 108b. 